The invention relates to a process for extruding polymer melts to form hollow chamber sections by pressing the polymer melt through a heated shaping tool with an internal shaping mandrel, wherein shaping tool and shaping mandrel determine the outer and inner contours of the hollow chamber section, and by subsequently sizing and cooling the hollow chamber section strand exiting from the shaping tool in a sizing and cooling unit. In addition, the invention relates to a device for carrying out such a process, comprising a shaping tool which contains an internal shaping mandrel and a sizing and cooling unit.
Economic aspects very often require hollow chamber sections to be produced instead of solid sections because material can be saved by this and a greater production speed can be used. However, in the case of hollow chamber sections greater requirements have to be met with respect to measurement, shape and functional accuracy and these are not always easy to fulfill. In the case of known processes and devices for extruding polymer melts to form hollow chamber sections (Michaeli r Kunststoffe", 1991, in particular pgs. 194 to 197) the following, restrictive conditions, in particular, have to be fulfilled inter alia: The profile cross section is intended to be kept as simple as possible, wherein interior webs are to be avoided if at all possible. The wall thickness of interior webs should be selected to be around 20 to 30% smaller than the wall thickness of the outer wall. The section should be designed such that it retains its shape for a short time after exiting from the tool, even in the plastic state. Material accumulations and cracks in the wall thickness are to be avoided since these make the control of the melt flow distribution in the tool difficult, and problems result during cooling (sunk spots on account of varying shrinkage and warping of the section). The cavities in the hollow chamber section should not be too small as, otherwise, the bodies (mandrels) displacing the melt are too small and cannot be adequately guided. Finally, the section axis is intended to be located in the worm shaft of the extruder used in order to keep flow path differences as small as possible.
For the production of the known hollow chamber sections, so-called monoextrusion tools are used, which are followed by sizing and cooling units as separate units in a spatially separated manner, wherein a vacuum is often used for the sizing. As a result of the spatial separation of shaping tool, on the one hand, and sizing and cooling unit, on the other hand, the shaped melt passes without pressure into the last-named unit and is there shaped and cooled without pressure and under vacuum. In the intermediate space between shaping tool and sizing and cooling unit, where ambient temperature and atmospheric pressure naturally prevail, the extruded hollow chamber section, which exits from the shaping tool and is intended to be as small as possible in order to be able to adjust it again in a simple manner in the sizing and cooling unit, will generally swell out.
All these restrictions and conditions result in the fact that up to now only relatively simple hollow chamber sections could be produced while, in the case of hollow chamber sections with complicated cross-sectional shapes, the requirements with respect to accuracy and dimensional stability could be met only with difficulty.
A related process is known from DE-24 34 381A1. This publication relates to the production of a hollow chamber section with closed cells, i.e. without axially continuous hollow chambers. The cooling obviously takes place not at strand extrusion pressure but at atmospheric pressure.
U.S. Pat. No. 3,182,108 discloses a process for extruding tubes from polymers having only one single hollow chamber, wherein in a sizing and cooling unit a wall made porous, for example, by openings abuts either on the outer or inner side of the extruded pipe, a degasification of the extrudate taking place through this wall. On account of the difference in pressure required for the degasification, the extrudate cannot be subject to extrusion pressure in the sizing and cooling unit.
It is known from U.S. Pat No. 5,132,062 to extrude a foamed polymer material to form simple hollow sections, for example to form tubing without axially extending inner webs. A polymer material cannot, as is well known, foam under strand extrusion pressure. For foaming, it is, on the contrary, necessary to relieve the pressure to such an extent that the foam can form in the shaped cavities available for this. For this reason, U.S. Pat. No. 5,132,062 also does not disclose any sizing and cooling of a hollow section strand under strand extrusion pressure since, in this case, only foamed polymer material is used. Only simple profile cross sections, for example tubes, can be produced with this known process.